In the handling of flowing aqueous products, such as natural gas, for example, it is necessary to accurately measure the quantity of gas that is flowing through a flow conduit system, typically referred to as a pipeline. In the natural gas industry, the gas is typically sold by quantity measurement, such as by volumetric measurement, where natural gas flows through pipelines at high velocity and under high pressure. An acceptable way to measure the flowing gas is by passing the flow stream across an orifice of predetermined size and measure the pressure differential that exists immediately upstream and downstream of the orifice. This differential pressure is utilized in conjunction with other factors, such as the pressure and temperature of the flowing gas, for purposes of calculating the volume of gas that flows through the pipeline and across the orifice. Typically, electronic means is utilized for substantially continuous detection and measurement of the flowing gaseous medium, thus providing both the seller and the purchaser with accurate data reflecting flow measurement.
The orifice is typically supported in the flow stream by means of a mechanical structure typically referred to as an orifice fitting. Orifice fittings may simply take the form of a body structure that is welded or bolted into the pipeline and which provides a receptacle for an orifice plate that defines the orifice. Because the orifice plate is subject to wear by line scale, sand and other particulate contained in the flowing gas, the orifice fitting must be provided with means facilitating removal and replacement of the orifice plate. Obviously, any wear of the orifice results in inaccurate measurement of the flowing gaseous medium and, therefore, replacement of orifice plates occurs frequently. Where the orifice fitting structure is provided with a simple bolted bonnet structure allowing insertion and removal of the orifice plate, it is, of course, necessary to discontinue flow through the pipeline and reduce line pressure to substantially atmospheric pressure in order to allow replacement of an orifice.
An orifice fitting mechanism has been developed that allows removal, replacement and reinsertion of orifice plates without necessitating reduction of line pressure and without interrupting flow. This development allows frequent inspection of the orifice plates as well as promoting orifice plate replacement without interfering with the production of the pipeline system. One such orifice fitting structure is that manufactured and sold by Daniel Industries, Inc. as the "Senior" orifice fitting which is exemplified by U.S. Pat. No. 1,996,192.
Orifice fittings that promote removability and replacement of orifice plates without intefering with line production typically incorporate a body structure including upper and lower body sections. The lower body section registers with the pipeline structure and provides adequate support for the orifice plate. A valve mechanism referred to as a valve seat and valve strip is located between the upper and lower body sections and provides a sealing mechanism. In operation, when the orifice plate is to be changed or inspected, it is raised from the lower body section through the valve seat and into the upper body section of the orifice fitting with the valve mechanism in its open position. With the valve mechanism in its open position, the upper and lower body sections will be balanced at line pressure. After the orifice plate has been raised into the upper body section, the intermediate valve mechanism will be closed, thus isolating the upper body section from the lower body section and line pressure. With the valve mechanism closed, the existing pressure in the upper body section can be vented to the atmosphere by opening a vent valve, thus reducing the upper body section to atmospheric pressure. After this has been accomplished, a clamping bar will be released at the upper extremity of the upper body section and an exit opening will be thereby defined through which the orifice plate assembly may be moved outwardly of the upper body section, thus positioning the orifice plate for ready inspection and/or removal.
The typical practice in the pipeline industry is to manufacture both the upper and lower body sections by conventional sand casting methods incorporating steel as the casting material. The steel casting method or manufacture is quite acceptable for the manufacture of smaller sized orifice fittings, but in larger sizes certain problems develop that are associated with foundry capabilities. A fundamental requirement of the body construction is that it provide a narrow air space between its two inwardly projecting circular bosses which define opposed circular sealing surfaces that establish sealing engagement with an elastomeric sealing element carried by the orifice plate assembly. The narrow air space requirement is caused by the standard practice of measurement which requires pressure tap holes on each side of the orifice plate to be quite closely located with respect to the orifice plate. In order to make this possible, the air gap between the internal circular body projections is reduced to a size range in the order of 1" to 11/4".
It is extremely difficult to cast a heavy body section in steel with an air space between the internal projections that is narrow enough so that satisfactory machining allowance can be removed from these internal projections and a cleanly machined surface be thus produced. The combination of extremely heavy metal sections and quite thin cored clearances within the casting structure develop requirements that are fundamentally incompatible. The heat present in the heavy steel sections tends to burn away the binders holding the sand grains in the cores, thus causing erosion and deterioration of the cores by the flowing molten steel. As a result, the cores break down permitting undesirable introduction of the molten steel into those portions of the internal chamber which are intended to be kept clean for passage of the orifice plate carrier therebetween. The sand that becomes eroded from the cores typically becomes incorporated into the metal of the closely spaced sections and subsequent machining of these sections will expose the sand contamination.
One solution to the casting problem that is developed by the requirement for unreasonably narrow air space between internal projections in the body concerns casting the body structure without an internal air space groove. With this form of casting, a continuous through conduit exists in the casting structure and a suitable narrow air space groove can be subsequently machined in this continuous conduit, thus developing opposed sealing surfaces that are engaged by a circular elastomeric sealing element supported by the orifice plate assembly. This practice introduces unsatisfactory complications for the foundry, however, since the cored internal chambers of the body structure which provide clearance for a plate carrier to operate so as to raise and lower the orifice plate, require proper positioning and adequate support when the cores are placed in the foundry mold. Accordingly, most foundries insist on some connection between the main conduit core and the surrounding rectangular cored sections which accommodate the carrier in the lower position.
Numerous devices are employed to mitigate the difficulties of supporting the rectangular clearance chamber cores, including external core prints, which are brought through the side walls of the rectangular chamber defined by the body structure. These core prints also serve for cleaning access for the removal of the burned core residue during the casting cleanup operation. Each such core print window must then be filled with a suitable cast steel plug which is welded into place to restore the chamber side wall to its continuous contour. All of these manufacturing operations that are performed on the body wall structure are costly and time consuming procedures which introduce undesirable long term delivery requirements from the foundry and also result in relatively high casting costs.
A second difficulty associated with the cast body structure of orifice fitting is that the machined surfaces of the through bore and the internal opposed sealing faces of the inwardly projecting hubs tend to show foundry defects, such as porous areas, cracks, shrinkage voids, etc. The tramp sand which is scoured off the walls of the casting mold by the hot molten steel is apt to be carried into sections of the casting where cleanly machined surfaces are required in the final product. For accurate flow measurement, it is necessary that the through bore of the orifice fitting have a high surface finish and that it be cylindrical within extremely close tolerances. Obviously, any surface imperfections that occur because of sand inclusions that are exposed by the machining operation requires weld repair and subsequent remachining. Such operations obviously increase the manufacturing cost of the product and, thus, detract from its competitive nature.
When casting imperfections occur in the opposed sealing faces of the internal hubs of the body structure, they are very difficult to repair because of the limited access that is provided by the narrow air space between the hub faces. Accordingly, the repair of such defects is an uncertain procedure, subject to secondary repairs when these surfaces are finished machined.
The difficulties described in connection with manufacture of the lower body section of large orifice fittings have their counterparts when considering the casting procedures employed for manufacture of the upper body sections. The upper body section is customarily produced as a steel casting that is formed with a narrow cast slot at its upper and lower extremities to permit the through passage of the orifice plate carrier structure. There is a somewhat relieved chamber in the central portion of the upper body section. In foundry practice, the necessity for providing extremely narrow upper and lower slots during the casting process results in the development of a core that is insufficiently strong to resist the combination of fluid pressures across wide spans and the burning action of the molten steel. Obviously, both of these problems tend to become worse as the size of the casting structures become larger because of the sheer volume of the molten steel that is employed for pouring the larger size castings. In practice, the foundries have insisted upon dividing the upper body section into two upper body pieces so as to reduce the unsupported span of the interior core for larger size castings. This introduces an economic penalty in the extra machine work that is required to develop a sealed connection between the two halves of the upper body section.
In typical orifice fitting structures, including a valve assembly between upper and lower body sections, it is typical for the valve assembly to be secured preferably to the upper body section by suitable means of connection. As the orifice fitting structure is placed into service and is subjected to relatively high internal pressures, pressure deflection of the upper body section is transmitted to the valve mechanism and, in some cases, interferes with the sealing ability of the valve mechanism. Even though sealant material may be injected into seal grooves to assist in the development of a positive seal, the structural distortion that may be developed due to pressure deflection of the upper body section can, in some cases, prevent the development of an efficient seal. When this occurs, it may not be possible to vent the upper body section rapidly enough to develop a high pressure differential across the valve mechanism. Typically, the sealing ability of the valve mechanism is enhanced by the pressure differential and, unless the pressure differential is sufficiently developed, the valve mechanism may not seal properly. Thus, it is important to insure that rapid upper body venting will occur.
Regardless whether the orifice fitting is of cast or fabricated construction, it is desirable to insure that a seat mounting plate be provided that is free of distortions that might occur due to the internal forces that are developed by service pressure. This is important because of the desirability that the seat structure present a true, undistorted surface for development of efficient sealing engagement with the valve strip of the orifice fitting. It is also desirable to provide a seat mounting plate structure that renders the orifice fitting structure field repairable to a large extent, especially where the orifice fitting mechanism is subjected to a highly corrosive environment, such as under service conditions where the fluid medium has a high level content of hydrogen sulfide. This type of service is typically referred to in the industry as "sour gas service" and it is well known that sour gas is extremely corrosive to metal parts that are maintained under stress. For example, hydrogen embrittlement, which is also referred to as stress corrosion, will occur quite readily when stressed metal parts are subjected to natural gas having a high hydrogen sulfide content. It is desirable to provide a seat plate structure for orifice fitting mechanisms wherein the seat plate is not only substantially free from the stresses applied to the body structure by pressure induced forces, but is also easily removed and replaced in the event of corrosion under corrosive service conditions.
In view of the foregoing, it is a primary feature of the present invention to provide a novel orifice fitting structure having upper and lower body sections of fabricated steel plate construction and which is functionally satisfactory for the service conditions that are intended.
It is also a feature of the present invention to provide a novel orifice fitting construction that establishes the capability of producing large size orifice fittings and a simple construction which can readily accommodate and wider spans required for upper body sections without necessitating division of the upper body sections into more than a single unitary structure.
It is a further feature of the present invention to provide a novel orifice fitting structure that includes a lower body section in which the wrought steel plate employed includes a sound weldment which can be machined simply with a certainty of clean sound metal surfaces for the critical sealing areas and for the critical through conduit bore.
Another feature of the present invention contemplates the provision of a novel orifice fitting structure that provides for the use of lower cost construction material and the use of steel plates to develop body sections for orifice fittings that effectively replace the more expensive steel castings that are ordinarily utilized.
It is another feature of the present invention to provide a novel orifice fitting structure that provides the manufacturing facility with the capability of prompt delivery of large size products that are typically made to special order of the customers and, thus, promoting competitive advantage over the manufacturers of similar products where extremely long delivery time is necessitated by the number and difficulty of the various manufacturing processes for such products.
It is an even further feature of the present invention to provide a novel orifice fitting structure that effectively provides for the introduction of the manufacturing process to other countries where foundry capacity is limited and similarly to provide for effective low cost manufacture under circumstances where relatively small quantities of large size products are typically ordered by customers.
It is also a feature of the present invention to provide a novel fabricated orifice fitting structure where products may be effectively manufactured and sold under profitable conditions even when order quantities are sufficiently low that the purchase of patterns and the attendant cost of developing satisfactory foundry practices, such as heading, gating, etc., is uneconomic.
Another feature of the present invention is a novel orifice fitting structure incorporating an intermediate "seat mounting plate" between upper and lower body sections which serves as a stable mounting surface for the valve seat and maintains the seat mounting plate free of distortions that are typically associated with the lower face of the upper body section due to the forces developed by service pressure.
It is also a feature of this invention to provide a novel orifice fitting structure incorporating a seat mounting plate structure which permits the field replacement of a valve seat without regard to deterioration which may have occurred on the upper body section during its service life and without regard to replacement of either of the upper or lower body sections.
Another important feature of the present invention concerns the provision of a novel method of manufacturing fabricated orifice fitting structures whereby all body structures include both interior and exterior welds for the purpose of facilitating the structural integrity that is necessary for high pressure service conditions.
Other and further objects, advantages and features of the present invention will become apparent to one skilled in the art upon consideration of this entire disclosure. The form of the invention, which will now be described in detail, illustrates the general principles of the invention, but it is to be understood that this detailed description is not to be taken as limiting the scope of the present invention.